Histamine [2-(imidazol-4-yl)ethylamine] is a transmitter substance. Histamine exerts a physiological effect via multiple distinct G-protein coupled receptors. It plays a role in immediate hypersensitivity reactions and is released from mast cells following antigen IgE antibody interaction. The actions of released histamine on the vasculature and smooth muscle system account for the symptoms of the allergic response. These actions occur at the H1 receptor (Ash, A. S. F., and Schild, H. O., Br. J. Pharmacol. 1966, 27, 427-439) and are blocked by the classical antihistamines (e.g., diphenhydramine). Histamine is also an important regulator of gastric acid secretion through its action on parietal cells. These effects of histamine are mediated via the H2 receptor (Black, J. W., et al., Nature 1972, 236, 385-390) and are blocked by H2 receptor antagonists (e.g., cimetidine). The third histamine receptor —H3— was first described as a presynaptic autoreceptor in the central nervous system (CNS) (Arrang, J.-M., et al., Nature 1983, 302, 832-837) controlling the synthesis and release of histamine. Recent evidence has emerged showing that the H3 receptors are also located presynaptically as heteroreceptors on serotonergic, noradrenergic, dopaminergic, cholinergic, and GABAergic (gamma-aminobutyric acid containing) neurons. These H3 receptors have also recently been identified in peripheral tissues such as vascular smooth muscle. Consequently there are many potential therapeutic applications for histamine H3 agonists, antagonists, and inverse agonists. (See: The Histamine H3 Receptor—A Target for New Drugs; Leurs, R., Timmerman, H., Eds.; Elsevier, 1998; Morisset et al., Nature 2000, 408, 860-864.) A fourth histamine receptor —H4— was recently described by Oda et al. (J. Biol. Chem. 2000, 275(47), 36781-36786).
The potential use of histamine H3 agonists in sleep/wake and arousal/vigilance disorders is suggested based on animal studies (Lin, et al., Brain Res. 1990, 523, 325-330; Monti, et al., Eur. J. Pharmacol. 1991, 205(3), 283-287). Their use in the treatment of migraine has also been suggested (McLeod, et al., Soc. Neurosci. Abstr. 1996, 22, 2010) based on their ability to inhibit neurogenic inflammation. Other applications could be a protective role in myocardial ischemia and hypertension where blockade of norepinephrine release is beneficial (Imamura, et al., J. Pharmacol. Expt. Ther. 1994, 271(3), 1259-1266). It has been suggested that histamine H3 agonists may be beneficial in asthma due to their ability to reduce non-adrenergic non-cholinergic (NANC) neurotransmission in airways and to reduce microvascular leakage (Ichinose, et al., Eur. J. Pharmacol. 1989, 174(1), 49-55).
Several indications for histamine H3 antagonists and inverse agonists have similarly been proposed based on animal pharmacology experiments with known histamine H3 antagonists (e.g. thioperamide). These include, dementia, Alzheimer's disease (Panula, et al., Soc. Neurosci. Abstr. 1995, 21, 1977), epilepsy (Yokoyama, et al., Eur. J. Pharmacol. 1993, 234, 129-133) narcolepsy, eating disorders (Machidori, et al., Brain Res. 1992, 590, 180), motion sickness, vertigo, attention deficit hyperactivity disorders (ADHD), learning and memory (Barnes, et al., Soc. Neurosci. Abstr. 1993, 19, 1813), schizophrenia (Schlicker, et al., Naunyn-Schmiedeberg's Arch. Pharmacol. 1996, 353, 290-294); (also see Stark, et al., Drugs Future 1996, 21(5), 507-520, and Leurs, et al., Prog. Drug Res. 1995, 45, 107-165, and references cited therein). Histamine H3 antagonists, alone or in combination with a histamine H1 antagonist, are reported to be useful for the treatment of upper airway allergic response (U.S. Pat. Nos. 5,217,986, 5,352,707, and 5,869,479). Recently, a histamine H3 antagonist (GT-2331) was identified and is being developed by Gliatech, Inc., (Gliatech, Inc., Press Release Nov. 5, 1998; Bioworld Today, Mar. 2, 1999) for the treatment of CNS disorders.
Background materials concerning histamine H3 ligands have been reviewed (Leurs, R., Timmerman, H., 1998). Within this reference the medicinal chemistry of histamine H3 agonists and antagonists is reviewed (see Krause, et al., and Phillips, et al., respectively). The importance of an imidazole moiety containing only a single substitution in the 4-position was noted together with the deleterious effects of additional substitution on activity. Particularly methylation of the imidazole ring at any of the remaining unsubstituted positions was reported to strongly decrease activity.
More recently several publications have described histamine H3 ligands that do not contain an imidazole moiety. For example, Ganellin, et al., Arch. Pharm. (Weinheim, Ger.) 1998, 331, 395-404; Walczynski, et al., Arch. Pharm. (Weinheim, Ger.) 1999, 332, 389-398; Walczynski, et al., Farmaco 1999, 684-694; Linney, et al., J. Med. Chem. 2000, 43(12), 2362-2370; Tozer and Kalindjian, Exp. Opin. Ther. Patents 2000, 10(7), 1045-1055; U.S. Pat. No. 5,352,707; PCT Application WO99/42458, Aug. 26, 1999; and European Patent Application 0978512, Feb. 9, 2000.
Imidazole derivatives, such as acylimidazole derivatives, are reportedly useful in the preparation of drug carrier systems. For example, dextrans have been used as carriers for flurbiprofen (CAS Reg. 51543-38-5), a nonsteroidal anti-inflammatory prostaglandin synthesis inhibitor, more specifically, a non-selective inhibitor of the cyclooxygenase activity of prostaglandin H synthase (PGHS). To prepare a polymeric drug carrier for flurbiprofen, conjugates of this drug are reportedly synthesized by making their acylimidazole derivatives, which are condensed in situ with dextrans of a variety of molar masses. The conjugated entities reportedly show a remarkable reduction in ulcerogenicity with respect to the parent flurbiprofen. S. K. Shrivastava, et al., Pharmazie 2003, 58(6), 389-391. Embodiments of synthetic methods developed in the context of this invention are useful in the preparation of acylimidazole derivatives.
Other acylimidazole derivatives that comprise a quaternary ammonium moiety are reportedly antagonists of tachykinins, including NKA (neurokinin A), NKB (neurokinin B), and Substance P, acting at the human neurokinin-1 (NK1), neurokinin-2 (NK2), and neurokinin-3 (NK3) receptors. Such compounds can reportedly be used for treating an inflammatory disease, a central nervous system (CNS) disorder, a gastrointestinal (GI) disorder, a urogenital tract disorder, a pulmonary disorder, an allergy, a hypersensitivity disorder, a proliferative disorder, a vasospastic disease, a fibrosing or collagen disease, reflux sympathetic dystrophy, an addiction disorder, a stress-related somatic disorder, a peripheral neuropathy, a neuropathological disorder, a disorder relaed to immune enhancement or suppression, a rheumatic disease, and an ophthalmic disease. See U.S. Pat. Nos. 6,207,678 B1 and 6,380,396 B1. Embodiments of synthetic methods developed in the context of this invention are useful in the preparation of acylimidazole derivatives that comprise a quaternary ammonium moiety.
Several publications have described the synthesis of functionalized imidazole compounds. Examples include U.S. Pat. Nos. 6,207,678 B1 (cols. 5-12, and preparations referred to in the Examples therein) and 6,380,396 B1 (cols. 5-12, and preparations referred to in the Examples therein) as well as a publication by Imbach, J. L., et al. (J. Heterocycl. Chem. 1967, 4, 451-454). 2-Chloroimidazoles can be useful intermediates in the preparation of functionalized imidazole derivatives and their preparation has been reported in the literature (Imbach, J. L., et al. 1967). Imidazole derivatives are important products because some of such products are pharmacologically active.
However, and as summarized below, conventional methodologies have synthetic disadvantages concerning one or more of these characteristics: Yield, scalability to multi-gram synthesis, regioselectivity, highly reactive by-product formation, impurity formation, number of synthetic steps, reaction conditions, and purifications. For example, some conventional methods rely on as many as eight synthetic steps, and/or are implemented under hazardous reaction conditions, and/or include reagents that are not desirable for large-scale work, and/or generate highly reactive by-products, and/or need chromatographic purifications.
For example, direct chlorination of N-alkylimidazole is not regioselective. Conventional methodologies to prepare the 2-chloro derivative rely on metal-halogen exchange between 2-imidazolyl lithium or a Grignard reagent, and a positive chlorine source. Two procedures that apply this strategy have been reported. In one of such procedures, (Boga, C., Del Vecchio, E., Forlani, L, Milanesi, L., and Todesco, P. E., J. Organomet. Chem. 1999, 588, 155-159), 1-methylimidazole is treated with n-BuLi to generate 2-imidazolyl lithium and then with trichloroacetyl chloride according to the following scheme:

Reactions according to this methodology, however, produce a highly reactive dichloroketene as a by-product, which undergoes decomposition under the reaction conditions. This decomposition adversely affects product recovery, and the reaction is characterized by a yield that is too low to allow for its implementation in a scale-up process.
In another of such procedures (Boga, C., Del Vecchio, E., Forlani, L., and Todesco, P. E., J. Organomet. Chem. 2000, 601, 233-236), carbon tetrachloride is used as the halogen source instead of trichloroacetyl chloride, according to the following scheme:

As evinced by the reaction scheme, this methodology also leads to the generation of highly reactive by-products during the reaction. The dichlorocarbene produced during the reaction leads to the formation of polymeric impurities, which adversely affect the isolation of the product of interest in large-scale reactions, such as reactions for the production of more than, for example, 10 g of product.
Yields for the 2-chloro derivative of about 20%-40% have been reported for these conventional methodologies that employ reagents such as trichloroacetyl chloride or carbon tetrachloride. Yields of less than 10% for the same derivative are obtained when Cl2 is used instead of trichloroacetyl chloride and carbon tetrachloride.
Highly reactive intermediates, such as dichloroketene and dichlorocarbene, are believed to participate in side reactions and undergo decomposition, making efficient conversion and product isolation unattainable in large-scale processes. Yet, large-scale processes are desirable for the synthesis of imidazole derivatives, including derivatives that are pharmacologically active, such as compounds that exhibit histamine H3 receptor modulating effects, comprising histamine H3 receptor antagonists.
In contrast with conventional methodologies, embodiments of the present invention provide synthetic methods that are characterized by at least one of the following features: High yields, with embodiments producing at least about 90% yield of the intermediate imidazole derivative of interest; regioselectivity; small number of synthetic steps, with embodiments having as few as two synthetic steps, such as regioselective substitution of the imidazole ring C-5 position with the use of a C-2 position blocking agent that is also a reactive leaving group in the subsequent substitution of such C-2 position; use of readily available and inexpensive reagents, such as perhaloalkanes and N—F electrophilic fluorinating agents as sources of halogen; stable by-product formation under synthetic reaction conditions; easy and reliable removal of by-products, such as removal of perhaloalkenes by distillation; purification by simple procedures, such as crystallization; and suitability for the production of multi-gram quantities of the products of interest.
A series of heterocyclic derivatives with the ability to modulate the activity of the histamine receptor, specifically the H3 receptor, are described herein. Also described are methods for preparing these heterocyclic derivatives, and other imidazole derivatives, that are amenable to large-scale procedures. Synthetic methods described herein are useful in the preparation of such H3 receptor modulators; embodiments of synthetic methods developed in the context of this invention are also useful, as indicated above, in the preparation of a variety of imidazole compounds in addition to such H3 receptor modulators.